The table hill volcanics of the officer Basin—Precambrian or Palaeozoic?
Titel:
The table hill volcanics of the officer Basin—Precambrian or Palaeozoic?
Auteur:
Compston, W.
Verschenen in:
Australian journal of earth sciences
Paginering:
Jaargang 21 (1974) nr. 4 pagina's 403-411
Jaar:
1974
Inhoud:
The Table Hill Volcanics of the Officer Basin were first dated as approximately 1100 m.y. from Rb-Sr model ages for total-rock samples of basalt from the Yowalga No. 2 bore. Later regional mapping, however, places the Volcanics as Marinoan (very late Precambrian) or younger, and receives support from discordant K-Ar ages ranging from 330 m.y. to 445 m.y. New total-rock analyses confirm the original Rb-Sr data, but analyses of separated minerals do not confirm the low value for the initial 87Sr/86Sr that had been assumed to calculate the 1100 m.y. model age. Instead, apparently-unaltered primary pyroxenes indicate that the initial 87Sr/86Sr could be as high as 0.718. Combined with the total-rock results, this yields an apparent age for the basalt of 575 ± 40 m.y. It is possible in principle that the high 87Sr/86Sr in the pyroxenes could be due to Sr isotope exchange during a Palaeozoic metamorphism, but there is absolutely no field or petrological evidence for such an event. Consequently, and in view of the stratigraphic evidence for their age, the Rb-Sr data are best interpreted as signifying an original extrusion of the basalts at 575 ± 40 m.y., together with a prehistory of the magma that includes contamination with radiogenic Sr and alkalis from Precambrian crustal material.
Tijdschrift beschrijving